Viruses such as measles, influenza, and HIV cause considerable morbidity worldwide. New emerging viruses are a constant threat because of the speed of their spread and their potential to cause widespread disease before therapies can be developed. Not much is known about innate immune responses against viruses. The immune signaling pathways in Drosophila are highly conserved with immune pathways in humans, worms, and plants, making it an excellent model for study. We have established a system to examine antiviral innate immune responses using the dsRNA birnavirus, Drosophila X virus (DXV) infection of Drosophila melanogaster. We have identified the Toll and RNAi pathways as important for host defense. From a forward genetic screen, we have also identified novel mutations important for viral pathogenesis or the antiviral immune response. The specific aims of this proposal are: 1. To study the roles of the Toll and RNAi pathways in the antiviral response. We will investigate whether these pathways interact or act independently, and determine whether dsRNA may be the pathogenic determinant that activates the antiviral immune response in flies. 2. To determine whether the new DXV-susceptible mutants are acting in the Toll, RNAi, or JAK/STAT pathways. 3. To determine whether an overactive immune response is being suppressed in the DXV- resistant mutants that show high titers of DXV. An overactive immune response appears to be harmful to the fly during DXV infection. We will determine the possible role of the Toll pathway in this response and characterize the DXV-resistant mutants to potentially identify negative regulators of this pathway. 4. To identify by map-based positional cloning two novel antiviral genes that affect either the Toll or RNAi pathway. Overall, the proposed experiments may give insight to new pathways, or new genes in Toll or RNAi pathways that are important for antiviral immune responses. Viruses come in many varieties and are successful in causing disease in humans; the Toll pathway and RNA interference (RNAi) are two means that the host uses to combat viral infection. We will characterize the role of these two pathways and identify novel genes important for the antiviral immune response.